The separation of a web from the forming wire in a forming section of a paper making machine and its transfer to a transfer felt is a critical step in the paper making process since this separation and transfer significantly influences the reliability and operation of the paper making machine as a whole. It is well known that in paper making machines which operate at low speeds, it is possible to use an open draw based upon the difference in speeds and travel of the web carrying forming wire and the transfer felt to transfer the web from the forming wire to the felt.
However, in paper making machines which operate at relatively high speeds which manufacture thin and consequently low strength paper, it is necessary to use a closed draw in the separation of the web from the forming wire and its transfer to the transfer felt.
Closed draw arrangements for separating a web from the forming wire and transferring the same to the press section of the paper making machine generally include a substantially continuous loop of transfer felt belonging to the press section of the paper making machine which is guided into contact with the web carried by the forming wire. The transfer felt is urged against the web carried by the forming wire such as by a rotating roller where upon the web adheres to the transfer felt and is thus transferred from the forming wire to the transfer felt and continues on to the press section of the paper making machine.
Generally, there are two types of closed systems for accomplishing the transfer of the web from the forming wire to the transfer felt. More particularly, the simpler of these arrangements, commonly termed "lick up transfer" utilizes a wet transfer felt which contacts the web to "lick up" the web by adhering the web to the surface of the transfer felt due to its wet nature. The other type of closed transfer system is generally termed a "vacuum pick up" wherein a vacuum ensures the adherence of the web to the transfer felt. A vacuum pick up system is generally more desirable than a "lick up" system since the former provides a greater choice in the selection of the quality of the transfer felt. In this regard, where the transfer felt also functions as a press felt, certain requirements must be taken into account in the choice of the felt, namely, the web should securely adhere to the surface of the transfer felt at the point of separation from the forming wire while at the same time the transfer felt must function in an efficient manner at the water removing press roll nip. These requirements, however, are often contradictory in that in order for the web to securely adhered to the felt at the separation station and to remain in adherence to the lower surface of the felt over a span between the pick up point and the first press roll nip, the felt must be relatively wet. However, as the moisture content of the felt is increased to facilitate adherence of the web to the felt, the dewatering capabilities i.e. the absorbency of the felt is correspondingly decreased, therefore rendering the water capacity at the press nip relatively inefficient. This is a distinct disadvantage in conventional vacuum pick up arrangements.
Vacuum pick up systems which utilize separate pick up suction rolls are known and are widely used in paper making machines. For example, such a system is used in a Fourdrinier Machine wherein the web is separated from the forming wire at a point located on the run of the forming wire between the chauffeur roll and the draw roll, the forming wire sloping during such run at an angle of about 45.degree. to the horizontal. The particular point at which the web is separated from the forming wire and is transferred to the transfer felt is determined by the particular design of the wire and press sections and their mutual locations. After the web is separated from the forming wire and becomes adhered to the transfer felt, the web carrying transfer felt wraps the pick up roll through a sector of about 45.degree. to 90.degree. where upon the web carrying the transfer felt moves onward to the press section.
Such vacuum pick up systems which utilize separate pick up suction rolls are often disadvantageous in that under certain conditions such as high speed paper making processes, the change of direction undergone by the web carried by the transfer felt on the pick up suction roll causes the web to loosen from the transfer felt due to the centrifugal forces acting thereon. In order to prevent this separation, the pick up roll is usually provided with a suction zone that extends beyond the actual zone wherein the web is separated from the forming wire. Although, this provision ensures that the web will remain adhered to the felt, the extension of the suction zone requires a corresponding considerable increase in the capacity of the vacuum system for the suction roller. Accordingly, systems of this type require a greater vacuum capacity than in arrangements where suction is only required to transfer the web from the forming wire to the transfer felt.
In order to at least partially alleviate some of the problems discussed hereinabove, a stationary transfer suction box has been utilized for separating the web from the forming wire rather than the use of vacuum pick up rolls. In this regard, U.S. Pat. No. 3,537,955 discloses a pick up arrangement for a paper making machine wherein a vacuum pick up shoe is provided for separating the web from the forming wire and transferring such web to a transfer felt for further processing. In such a device, the pick up shoe arrangement for removing a web from the forming surface of the forming wire includes an endless felt which is urged against the forming surface by means of a guide member having a curved surface in contact with the felt and a substantially planar surface diverging away from the felt thereby creating a pressure differential across the felt. However, with the pick up shoe described hereinabove, a significant vacuum source must be obtained in order to adequately transfer the web from the forming wire to the transfer felt.
In yet another paper manufacturing process, a transfer head or shoe including a convex facing surface having a transversely extending vacuum slot which communicates with a sufficient level of vacuum to effect transfer from the carrier fabric to the transfer felt is utilized. However, a significant force is required to impinge the transfer head against the transfer felt in order to effect sufficient transfer of the web from the forming wire to the transfer felt.
In yet another attempt to overcome the above-noted shortcomings, French Patent No. 1,573,109 discloses an automatic transferring apparatus for transferring a web from a forming wire to a transfer felt formed of a closed loop by means of a suction block mounted inside the felt loop and comprising a suction zone across the felt band which first contacts under pressure at a point of the transfer of the web carried by the forming wires so that a couching zone is established in which the three bands comprising the transfer felt, web and forming wire find themselves applied against the box. A perforated area of the suction box is provided to subject an inside surface of the transfer felt to a vacuum in order to effect transfer of the web from the forming wire to the transfer felt. However, again a significant force is necessary in order to transfer the web in the manner suggested.
Clearly, there is a need in the art for a web transfer device wherein a greater effective force can be generated in the couching zone for effecting sufficient transfer of the web from a forming wire to a transfer felt without jeopardizing the overall capacity of the paper making machine. Moreover, there is a need for a suction box construction wherein the overall production of the paper making machine is increased by increasing the pick up effectiveness of the transfer felt and reducing the number of breaks of the web during the paper making manufacturing process.